On-demand music service Spotify, which is currently available only in Europe, has been broadly praised by users (including me) for its large selection of music and exceptional responsiveness. Today, Spotify added two new levels of service: Open and Unlimited. The Open tier is more notable because, once again, it opens the service to users without an invitation.

The new levels are the latest step in Spotify's ongoing experiment to broaden its audience without compromising performance. When it launched in 2008, Spotify was free and offered unlimited streams to a PC, but an invitation was required. In February 2009, it opened the free PC-only service to all users in the United Kingdom. Demand was overwhelming, and last September the company went back to requiring invitations.

The two new tiers try to strike a balance. As explained in the chart at the bottom of this page, the Open tier is open to anybody in countries where Spotify is available, but limits users to 20 hours of music listening per month, with occasional advertisements to help pay the bills. If you want unlimited streams on your computer and can't seem to wrangle an invitation to the Free tier, you can sign up for Spotify Unlimited for 5 Euros or pounds per month, and you won't have to listen to ads. Spotify Premium, also ad-free, is required to stream songs to your iPhone or other mobile devices, and continues to cost 10 Euros or pounds monthly.

The service also launched in the Netherlands today. So when's it coming to the United States? According to Not-ify, a Tumblr blog devoted tracking the service's impending launch, it should be available in the U.S. by the end of 2010. That's good news for music listeners, although perhaps not for musicians: according to blogger The Cynical Musician, whose band has music on the service, Spotify's average payout per stream has been about one-tenth of one cent (not including payments for publishing rights, which are administered by third-parties such as BMI). That means users would have to stream your songs more than a million times per month before you could earn the minimum wage equivalent of $1,160. The scale of the free service, and its number of paying subscribers, will have to increase dramatically before Spotify can become the much hoped-for savior of the recording industry.